Demand for wideband wireless high-speed data services are on the rise. In a wideband wireless communications system, the signal tends to suffer from the frequency selective fading due to multi-path. Orthogonal Frequency Division Multiplexing (OFDM) systems have been proposed to combat the frequency selective fading by dividing the total bandwidth into a plurality of subcarriers where the bandwidth on each of multiple subcarriers is sufficiently narrow to enable the data modulation symbols carried by that subcarrier to experience relatively flat fading.
Orthogonal Frequency Division Multiple Access (OFDMA) systems use the OFDM modulation technique to multiplex the traffic data of a plurality of mobile stations (MS) in frequency and time. FIG. 1A illustrates that in an OFDMA system, the available radio resource over one time interval (frame) for traffic data, which may or may not include the radio resource occupied by the guard band, the control channels, the pilot or other overhead channels, is divided in time and frequency. The smallest bin 110 is one subcarrier in frequency over one OFDM symbol period in time. The forward shared scheduling channels (F-SSCH) transmit assignment messages that communicate which mobile station (MS) is assigned with which bin or group of bins for data transmission. In order to reduce the overhead of the F-SSCH, a plurality of subcarriers over a period of one frame, consisting of a plurality of OFDM symbols in time, are assigned to a mobile station for data transmission.
In a cellular network or an ad hoc network, some mobile stations may be moving at fast rate of speed with respect to the base station, while other mobile stations are more stationary when they transmit or receive data. Some mobile stations experience severe multi-path while others have a near line-of-sight channel with the base station antenna. Therefore, two types of assignment strategies have been proposed for an OFDMA-based wireless communications system.
Referring to FIG. 1A as an illustrative example, the total radio resource 100 over one frame is divided up for four different assigned mobile stations, indicated by different patterns 115, 120, 125, 130, respectively. The radio resource assigned for a particular mobile station in FIG. 1A is disjoint in frequency with equal spacing between the adjacent assigned subcarriers and disjoint in time with frequency offset hopping from OFDM symbol to OFDM symbol. This type of assignment is called distributed assignment. In general, the radio resource assigned to a user (mobile station) via a distributed assignment is disjointed in either time or frequency or both.
Referring to FIG. 1B as another illustrative example, the total radio resource 150 is divided up for four different assigned mobile stations, indicated by shade patterns 160, 165, 170, 175, respectively. The radio resource assigned for a particular mobile station in FIG. 1B is contiguous in both frequency and time. This type of assignment is called localized assignment.
In order to optimize the utilization of the radio resource in an OFDMA-based wideband wireless communications system, a method is needed to multiplex the data packets of multiple mobile stations with different channel fading conditions using minimal control overhead.